The invention concerns a device for confocal surface measurement in body cavities, especially for measuring the surface profile of teeth in the oral cavity, with a probe which can be introduced into the body cavity, a light source supplying the probe, a detector which collects a light signal, and a processor which digitizes the detected signal and processes it.
The invention is based on a method for measuring surfaces of any type or contour. Various processes for surface measurement are known in practice.
For example, a light sectioning sensor can project a line of light onto the object and observe it at an angle using a CCD camera. The geometric deformation of the line is measured. The height differences on the object can be computed from this deformation. By moving the object under the sensor--perpendicularly to the light line--and by repeated measurement of a profile, surface form can be measured or determined from a series of profiles.
The light sectioning sensor is indeed a simply designed and robust sensor, but the oblique lighting which it requires causes unilateral shading of steep surfaces. That causes asymmetries in the imaging, or inaccuracies. Furthermore, error can be introduced into the measurements because of scattering of light from various depths of, for instance, an at least partially transparent tooth material.
Also, a system is already known for measuring the surface profile of teeth in the oral cavity. This means consists of the principal components camera, monitor, and computer. This system is connected directly to a grinding system to produce an inlay. (See Dr. Klaus J. Wiedhahn in DENTAL MAGAZIN 1/95, "Cerec 2--a new epoch?".) For the known system for measuring the surface profile of teeth, the camera or probe is designed so that infrared light is passed though an oscillating grooved grating, and is then reflected from the tooth surface, which is coated with the white powder, TiO.sub.2. Then the light passes through a symmetric ray path to the CCD sensor in the camera. Four individual photographs are made at different grating angles in each sequence of photographs (0.2 second). The four individual images are computed to produce a three-dimensional image of the tooth. The three-dimensional "optical impression" obtained with the camera is presented on a high-resolution color monitor as a pseudoplastic image, and the image and structural data are processed in the image-processing computer and the built-in processors, and sent to the grinding unit.
The known process discussed here, and its hardware, have problems due to the fact that it is always necessary to coat the tooth surface with one or more powders to assure a distinct reflection at the tooth surface. Furthermore, the construction of the camera with its CCD sensor is expensive.
Finally, it is already known in practice that surfaces can be scanned with confocal microscopy so as to generate three-dimensional pictures of the surface. In this respect, it is only necessary to refer to Johann Engelhardt and Werner Knebel, "Konfokale Laserscanning-Mikroskopie" [Confocal Laser Scanning Microscopy] in 'Physik in unserer Zeit', Vol. 24, No. 2, 1993, and to D. K. Hamilton and T. Wilson, "Three-dimensional Surface Measurement Using the Confocal Scanning Microscope" in Applied Physics, B27, 211-213, 1982. With respect to a corresponding system--Leica TCS NT--we refer to the Leica brochure "The Confocal System, Leica TCS NT", where application in the dental area is mentioned, particularly on page 16. However, such a system is too large for use in a patient's oral cavity, and too costly to build, and thus too expensive in the confines of dental use.
If we ignore the disadvantages mentioned above, confocal microscopy is very specially suited to surface measurement of tooth surfaces, because this process images only those structures which are exactly in the focal plane of the microscope objective. Thus measurement errors due to the partially transparent tooth material are effectively avoided. To be sure, the method of reflection measurement with the usual confocal microscope fails at steep transitions or flanks if their angle is greater than the aperture angle of the objective, because then the reflection no longer enters the objective, and is lost for evaluation. (See P. C. Cheng and R. G. Summers in Confocal Microscopy Handbook, Chapter 17.)